Exosomes are cell-derived vesicles that are present in many and perhaps all biological fluids, including blood, urine, and conditioned media from cell cultures. The reported diameter of exosomes is typically between 30 and 100 nm, which, for comparison, is larger than LDL but significantly smaller than red blood cells. Exosomes are known to be released from cells when multivesicular bodies fuse with the plasma membrane or when they are released directly from the plasma membrane. It is becoming increasingly clear that exosomes have specialized functions and play a key role in, for example, coagulation, intercellular signaling, and waste management. Consequently, there is a growing interest in the clinical applications of exosomes, including synthetic exosomes which recapitulate aspects of cell-derived exosomes. Exosomes can potentially be used for prognosis, therapy, and biomarkers for health and disease.